Heretofore, a parallel flat plate structure in which a flat plate-like anode and a flat plate-like cathode are opposed in parallel to each other has been known as a structure of a reactor which a gas reforming device (hereinafter, also referred to as a “plasma reformer”) includes, the gas reforming device activating process gas by non-thermal equilibrium low-temperature plasma and generating reform gas (Patent Document 1). In general, the anode and the cathode are embedded in flat plates as dielectrics. An electric discharge is generated between the anode and the cathode, which are embedded in the flat plates as the dielectrics.